Recording waveform for mark-length modulation optical recording

ABSTRACT

A method of recording information by changing a recording medium through irradiation with an energy beam such as a light beam, electron beam or the like. A single pulse or a plurality of pulses are employed for forming recorded dots. The pulse has a pulse width shorter than a time required for the center of energy beam spot to move from one to other end of the dot upon recording thereof. By virtue of the possibility of reversible change even in a recording film susceptible to high rate speed of change, the rate of information transfer can be increased. Overwrite recording can also be accomplished with the single laser beam. An increased amount of information can be handled in the recording and reproduction.

This application is a continuation, of U.S. application Ser. No.07/185,690, filed 4/22/88 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method of recording information byusing a recording medium which allows information to be rewrittenthrough irradiation with an energy beam such as a light beam, electronbeam or the like. More particularly, the invention is concerned with aninformation recording method which can be effectively applied to a phasechange type disc adapted for recording, erasing and rewriting by using alaser beam.

As methods of recording/erasing information on or from the phase changetype optical disc recording medium, there is known a method disclosed,for example, in Japanese Patent Application Laid Open No. JP-A-59-71140.According to this known method, erasing through crystallization ofinformation recorded already on a recording film thereof is accomplishedby maintaining the recording film for a relatively long period at atemperature at which the recording film can be crystallized throughirradiation of a light spot of elliptic shape having a major axisextending trackwise. Subsequent recording of new information is realizedby irradiating an adequately focused circular light beam modulated bythe information signal to be recorded. Recently, in the course ofstudies conducted by the inventors of the present application in aneffort to improve the material for the recording film, a method wasdeveloped which allows the recording film to be crystallized by means ofan adequately focused circular light spot moving on the disc. Morespecifically, information on the disc could be erased with the circularlight spot during one rotation of the disc, and subsequently recordingcould be accomplished during the single succeeding rotation of the discthrough irradiation with the laser beam having power modulatedcorrespondingly. Further, by modulating the laser power in accordancewith the information signal between the crystallizing power level andthe amorphizing power level, rewriting of the information could beaccomplished in the course of the single revolution of the disc.

However, in the case of the abovementioned technique, rate or speed ofthe change in atomic arrangement (e.g. crystallization) in the recordingfilm has to be further increased when the rotating speed of the disc isincreased with a view to realizing higher information transfer rate. Inthat case, even when the crystalline material is molten underirradiation by the energy beam (e.g. laser beam), the atomic arrangementresumes the original state (e.g. through recrystallization) duringcooling after the irradiation, making it impossible to bring about thechange in the atomic arrangement in the reverse direction (e.g.amorphization).

SUMMARY OF THE INVENTION

An object of the present invention is to provide an informationrecording method which is essentially immune to the problems of theprior art described above and which allows the phase change to occurreversively even when a recording film susceptible to high rate of phasechange is employed.

The above object can be accomplished by forming recorded dots with asingle pulse or a plurality of pulses of an energy beam rising to ahigher power level (e.g. amorphizing power level) having a medium (e.g.crystallizing) power level before forming the recorded dot and the pulsewidth or duration which is shorter than a time taken for the center ofthe energy beam spot to move from one to the other end of the recordeddot. The beam pulse may preferably have a pulse duration shorter than3/4 of the time that it takes for the light spot center to move from oneto other end of the dot upon recording thereof, more preferably shorterthan 1/2 and most preferably shorter than 1/4.

By making the pulse width shorter (narrower) as described above, heatdissipation due to heat conduction from the irradiated dot location tothe ambient region can be prevented, whereby beam energy for theirradiation can be lowered relatively. And more over, deformation ofreproduced signal due to heat accumulation even when high power levelcontinued for a long time can be reduced.

According to the present invention, when overwriting of information(i.e. rewriting through overwriting without preliminarily erasing therecorded information) is performed with a single laser beam, the laserpower is not lowered to the power level for reading-out of informationat least approximately over the whole period of the overwritingoperation. Thus, the problem that the rate of quenching after the laserbeam irradiation tends to be lowered can be solved to great advantage.

The recording medium which can be employed in carrying out the inventionmay be formed of a material capable of undergoing a crystal-amorphousphase change. Additionally, it may be a material susceptible to othertypes of changes in atomic arrangement or electron spin. By way ofexample, the material capable of crystal-to-crystal phase change inatomic arrangement which requires quenching as well as the materialsusceptible to amorphous-to-amorphous phase change in atomic arrangementand magneto-optical recording materials can be employed. The method ofthe present invention is more effective for phase change recording mediathan for magneto-optical recording media. In this conjunction, it shouldbe understood that the crystallizing power level as mentioned abovemeans not only the power level required for crystallization of amorphousmaterial but also means a lower power level at which the phase changementioned above can take place.

The effectiveness of the method according to the invention isindependent of the type of energy beam used. In other words, a lightbeam, an electron beam, an ion beam and others can used. It should,however, be mentioned that in case the electron beam or an ion beam isemployed, a protective film deposited on the recording film of therecording medium should be preferably in thickness less than 1 μm andmore preferably less than 1000 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a view for illustrating the principle underlying the informationrecording method according to the present invention.

FIG. 2 is a sectional view of a recording medium on which information isto be recorded according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention will be described in conjunction with an exemplaryembodiment thereof.

The structure of a recording medium will be first explained withreference to FIG. 2.

A recording film containing In and Se as main components and susceptibleto a information signal recording and erasing by virtue of a reversiblephase change between a crystalline state and an amorphous (orapproximately amorphous) state and having both surfaces coated withprotective films of SiO₂ in a sandwiched structure is formed on asurface of a disc-like glass substrate having an ultraviolet light curedresin layer deposited thereon as shown in FIG. 2. Tracking grooves andpits indicative of addresses are copied on the surface of theultraviolet light cured resin layer. Subsequently, another ultravioletlight cured resin is applied over the abovementioned protective film,the multi-layer structure thus prepared being then bonded to anotherglass substrate and cured under irradiation of ultraviolet light.

Subsequently, the optical disc manufactured as described above isrotated at a speed of 600 rpm, and a location to be recorded is searchedwhile performing the tracking and the auto-focusing. At the location tobe recorded, power of the laser light beam is increased from a readingpower level to a crystallizing level, being followed by modulation ofthe power in such a manner as illustrated in FIG. 1. In this figure, anarray of amorphized dots formed on a recording track is shown at thetop. The remaining portion of the track is in the crystallized state.The inter-track regions remain in the state as deposited. In practicalapplications, position of the light spot is immobile, but a point or doton the disc is moved to the left as viewed in FIG. 1. However, forconvenience of illustration, the figure is so depicted that the dot onthe disc remains immobile with the light spot being moved to the right.In the graphic representation shown at the bottom in FIG. 1, the centerposition of the light spot which is assumed to move rightward is takenalong the abscissa while the level of laser power is taken along theordinate. At the dot region which is to be amorphized in accordance withthe information signal, the laser power is increased to an amorphizinglevel only for a short time. The time span during which the laser poweris increased to the amorphizing level is about 1/2 of the time requiredfor the center of the light spot to move from one to other end of acorresponding dot-like region to be amorphized (or dot-like region to bedegraded in crystallinity). The time span during which the laser poweris increased is defined as to width at a half of height of the laserpulse rising up from the crystallizing level to the amorphizing level.As the result of irradiation with the laser beam having the amorphizinglevel, the irradiated dot-like region is molten and subsequentlyquenched to become amorphous. In case the amorphized dot-like regionwhich is twice or more as long as that of the shortest amorphous regionis required to be formed, the inherent recording waveform can be dividedinto a plurality (three in the case of the illustrated example) of shortirradiation pulses, as is shown in FIG. 1 at a center portion thereof.The gap between these pulses is more preferably less than 1/2 of thepulse width. Consequently, regions resulting from such plural pulseirradiation form an elongated amorphous region. When such divided shortpulses are employed, the irradiation power should preferably be madelower than the crystallizing power level between the pulses. Morepreferably, the power level should be decreased to zero or the readinglevel during the inter-pulse period. Further, the laser power may bedecreased down to the crystallizing level or slightly higher level independence on the composition of the recording film and that of theprotective film. Magnitude of decrease in the laser power shouldpreferably be increased as the inter-pulse distance becomes narrower. Inthe exemplary case illustrated in FIG. 1, the laser power is lowered tothe reading level. Through irradiation with the plural divided pulses,there can be formed on the disc such a recording pattern which canensure reproduction of the signal with high fidelity to the originalsignal even when the latter includes a pulse signal component having along pulse width. When the information signal to be recorded includesthe pulses corresponding to shorter recorded dot length than 1.2 μm, thepulse division mentioned above can of course be spared. More preferably,the power level between two pulses corresponding to recorded dots ofshort distance, for example less than 3 μm, is lowered as shown in FIG.1 by a dotted line. The laser power waveform employed in theillustrative embodiment of the invention is so selected as to allow therewriting of information through the overwriting without need for thepreceding erasure. However, it is equally preferable that information isat least partly erased by preceding (crystallizing or amorphizing)irradiation before the irradiation in this embodiment.

The width of the pulse to the amorphizing level should preferably beselected equal to 1/2 of the time taken for the center of the light spotto move from one to other end of the dot-like region to be amorphized.More preferably, the pulse width of concern should be 1/4 or shorter forrealizing the satisfactory amorphization.

It is preferred to shorten the pulse width similarly when the recordingis performed with the laser light beam modulated between the readinglevel and the amorphizing level after all the tracks have been onceerased through crystallization with a continuous laser irradiation. Inthis case, it is however noted that the cooling rate is inherently highsince the laser power level is lowered to the reading level during theinterpulse period. Consequently, the effect attainable with this processis not so significant as in the case of the embodiment operation.

In the latter case, the crystallizing power level may be so adjusted asto lie in the range of 30 to 95% of the amorphizing level. Then, thesignal can be reproduced regardless of the pulse width. More preferably,the crystallizing power level should be in the range of 55 to 90%.

In the foregoing description, it has been assumed that the recording isrealized by amorphization. However, it is equally possible to regardcrystallization as recording.

As will now be appreciated from the foregoing description, the high rateof information transfer can be accomplished because of possibility ofreversible change in the recording film even when the recording filmsusceptible to high rate of change is used. Further, the overwriterecording can be accomplished with the single laser beam. These featurescontribute advantageously to the recording/reproduction capability for alarge amount of information.

What is claimed is:
 1. A method of recording information on a recordingmedium, which is capable of achieving a first phase by application of anenergy beam of a first energy level and a second phase by application ofthe energy beam of a second energy level, by projecting on the recordingmedium pulses of the energy beam so as to record the information on therecording medium in a form of one or more dots where the recordingmedium takes the first phase, said method comprising the steps of:movingthe recording medium relative to the energy beam; and projecting on therecording medium the energy beam of the second energy level when no dotis to be recorded. projecting a single pulse of the energy beam havingthe first energy level and a predetermined pulse length when a shortdot, having a length less than a predetermined length, is to berecorded; and projecting a plurality of consecutive pulses of the energybeam each having the first energy level and said predetermined pulselength when a dot having a length not less than said predeterminedlength is to be recorded, said predetermined pulse length correspondingto a duration time which is shorter than a time interval required forthe energy beam, in its relative movement, to move from one end to anopposite end of the short dot to be recorded.
 2. A method of recordinginformation according to claim 1, wherein said pulse duration time isshorter than 3/4 of the time required for the center of said energy beamto move from the one to the opposite end of said dot to be recorded. 3.A method of recording information according to claim 1, wherein saidenergy beam has an energy level of not higher than any of said first andsecond energy levels between adjacent two of said plurality of pulses.4. A method of recording information according claim 1, wherein saidenergy beam has an energy level lower than any of said first and secondenergy levels between adjacent two of said plurality of pulses.
 5. Amethod of recording information using a recording medium which isvariable between two different phases and capable of entering a firstphase by application of an energy beam of a first power level and asecond phase by application of an energy beam of a second power level,wherein the information is recorded on the recording medium in a form ofone or more dots where the recording medium is in said first phase, saidmethod comprising the steps of:projecting an energy beam on therecording medium with the second power level; moving the recordingmedium relative to the energy beam; and modifying the energy beam in aform of a pulse having the first power level and continuing for a firsttime period when the information is to be recorded in a form of a shortdot on the recording medium, said short dot having a length less than afirst predetermined dot length, wherein said first time period is lessthan a time for moving the energy beam across the recording medium by adistance equal to said first predetermined dot length; wherein when theinformation is to be recorded in a form of a dot longer than said firstpredetermined dot length, the energy beam alternates between the firstpower level and a third power level, not higher than any of the firstand second power levels a plurality of times for a second time periodcorresponding to a length of the longer dot.
 6. The method of claim 5wherein said second power level is lower than said first power level. 7.The method of claim 5 wherein said first power level amorphizes therecording medium.
 8. The method of claim 7, wherein said second powerlevel is 30 to 95% of said first power level.
 9. The method of claim 5wherein said first time period is less than 1/2 of said the time formoving the energy beam across the recording medium by a distance equalto a length of said short dot.
 10. The method of claim 5 wherein theenergy beam at said second power level causes the recording medium to becrystallized.